Developing positively surface-charged carbon dots as “semiconductor electrolyte” for photo-electrochemical H2O2 production based on oxygen reduction

•Positively surface-charged carbon dots (CDs) function as electrolytes in PEC system.•CDs solution regulates PEC O2 reduction into H2O2 with high yield and selectivity.•CDs and photocathode unite into a dynamic p-n heterojunction for charge separation.•CDs adjust electronic structure and hydrophilic property of catalytic interface.•CDs incorporate active sites that reduce kinetic barrier for H2O2 generation.